There are more than 3,000 counties in the United States, but in presidential elections they are not all created equal. Just a tiny fraction of them truly matter. Those places will get the bulk of the presidential candidates’ time — and their surrounding media markets will get the bulk of the television ad spending.

In an election that will be decided by a relatively small group of pivotal counties within a relatively small set of swing states, POLITICO has selected 25 critical battleground counties that are poised to be difference makers, places that could have a material impact on the presidential election. These counties aren’t the only ones that matter — most swing states have a handful of critical counties, not just one or two — but these 25 stand out, either for their voting history, population size or traditional impact on swing-state election results.

Suburban Denver’s Arapahoe and Jefferson counties have moved in tandem for the past four elections. Like Jefferson (but slightly more populous), Arapahoe was once a GOP stronghold that voted for Obama twice. It has a slightly higher Hispanic population than Jefferson County — 19 percent, according to the Census — which makes it an uphill challenge for Donald Trump.

This suburban Denver county twice backed George W. Bush then twice supported Barack Obama. Both Donald Trump and Hillary Clinton need to shore up their bases here in a place where there are more unaffiliated voters than Republicans or Democrats and major party registration is at near parity.

Jacksonville’s Duval County has been a reliable Republican performer over the years, though it nearly went for Obama in 2008 — in large part due to high African-American turnout. While Bill Clinton was in town recently to open up a Democratic campaign office, party leaders are skeptical they can win here with Hillary Clinton on the ticket. That said, they will look to get out as many voters as they can here to help compensate for expected losses in other northern parts of the state.

This was George W. Bush territory in 2000 and 2004, but Obama brought it back into the Democratic fold in his two elections. At one end of the I-4 corridor, Tampa’s Hillsborough County is a top bellwether — since 1960, no candidate has won Florida without winning Hillsborough. Along with neighboring Pinellas County (St. Petersburg), the two counties include nearly a half-million registered Republicans.

Donald Trump is highly unlikely to win here — Democrats have a big voter registration advantage in this county where Trump owns one of his many homes. Trump’s local ties notwithstanding, Clinton is likely to run up her numbers here in the northernmost part of the South Florida region. The trick, for Trump, is to hold down her winning percentage here in a county that has nearly a quarter of a million registered Republicans. Republican county chairman Michael Barnett said he’d celebrate if Trump could do better than Romney’s 41 percent total here in 2012.

Democrats have a registration advantage in Iowa’s most populous county, and they are looking to run up Clinton’s numbers here as best as they can to offset expected losses in more conservative parts of the state. A Trump win is probably out of the question in a county that’s home to Des Moines and the state capitol — he finished third here in the caucuses behind first-place finisher Rubio and runner-up Cruz — but he will go a long way toward carrying the state if he can keep it close.

Eastern Iowa is often seen as Democratic turf — the last 4 Democratic presidential nominees have won Scott County. Neither Clinton nor Trump won here during the caucuses in this Mississippi River county. Home to Davenport, the state’s third-largest city, Scott County is also packed with independents who can swing either way. Trump is making a hard play here; His first visit to Iowa after accepting the nomination at the Republican National Convention was in Scott County.

MICHIGAN

Macomb County

2016 primary winners: Clinton, Trump

2012: Obama 52%, Romney 48%

Population: 864,840 (2015 est.)

Latest voter registration totals: 618,420
No party registration

The storied home of Reagan Democrats and the third most-populous county in the state, southeastern Michigan’s Macomb last went Republican at the presidential level in 2004. In 2008, Obama carried this county outside Detroit by 9 percentage points but that winning margin was cut in half in 2012 with Michigan native Mitt Romney leading the GOP ticket.

Oakland County

2016 primary winners: Clinton, Trump

2012: Obama 54%, Romney 46%

Population: 1,242,304 (2015 est.)

Latest voter registration totals: 937,155
No party registration

After Detroit’s Wayne County, Oakland County is the most-populous in the state. One of the highest income counties in the nation and a longtime GOP Midwestern stronghold, in recent years the suburban behemoth has begun trending toward Democrats — not even Mitt Romney, who grew up here, could win it. At the presidential level, Republicans haven’t won Oakland County since 1992, though George W. Bush twice came very close.

Clinton will win Democratic Clark County, the state’s population hub and home to Las Vegas. The question is just how big of a margin she can run up here — where roughly two-thirds of the state’s presidential votes were cast in 2012 — to offset Trump’s victories across much of the rest of the state.

The second-most populous county in the state, northern Nevada’s Washoe County is home to Reno. Once-reliably Republican — and essential to Republicans to help overcome the deficit they face in Las Vegas’ Clark County — Washoe voted twice for Bush and then twice for Obama.

The most populous county in the state and home to Manchester and Nashua, Hillsborough twice voted narrowly for Bush and twice for Obama. Republicans have a voter registration advantage here but in recent presidential elections, it’s had consistently tight margins: In 2012 Obama won it by less than 4,000 votes. In 2004 Kerry lost it by about 5,000 votes.

Since 1960, Democrats have won Rockingham County only three times. But it’s been a nail-biter in the past four presidential elections. Relatively few counties across the nation switched party support in 2012, but Rockingham was one of them, flipping from Obama in 2008 to Romney.

George W. Bush twice won comfortably in Forsyth County, the home of Winston-Salem. But along with the state’s other urbanized counties, Forsyth has moved to the left, twice backing Barack Obama. His win in 2012 was by a smaller margin in Forsyth County than in 2008, as the state flipped back to Mitt Romney.

Affluent, well-educated Wake County, home to Raleigh and part of the Research Triangle, was once Republican territory. But after twice voting for George W. Bush, it has since moved well to the left. Trump, who has tended to run better with voters without college degrees, got trounced in the primary in Wake earlier this year. He will need to cut into Clinton’s advantage here in North Carolina’s second-most populous county since Clinton is likely to roll up the score in Democratic Mecklenburg County (Charlotte).

OHIO

Hamilton County

2016 primary winners: Clinton, Kasich

2012: Obama 53%, Romney 46%

Population: 807,598 (2015 est.)

No party registration

A surge in African-American turnout helped deliver Cincinnati’s Hamilton County to Obama in 2008, making him the first Democratic presidential nominee since Lyndon Johnson to carry the population hub of southwest Ohio. Hamilton is ringed by heavily Republican Butler, Warren and Clermont counties, and it shares a border with Indiana, which is giving hope to GOP officials who think Trump’s running mate, Indiana Gov. Mike Pence, could marshal his supporters to cross the border and provide support. “Cincinnati is kind of considered a little bit of an island off the rest of Ohio. Some of the same messages that are going to work in Northeastern Ohio are maybe not going to work as well down here,” said county GOP Chairman Alex Triantafilou.

Lake County

2016 primary winners: Clinton, Kasich

2012: Romney 50%, Obama 49%

Population: 229,245 (2015 est.)

No party registration

Lake County is a perpetual nail-biter. It abuts Cleveland’s Cuyahoga County to the east and went narrowly to Mitt Romney in 2012. Republicans are bullish that the suburban county will stay in their column this time, in part because of frustration with Washington. GOP Chairman Dale Fellows noted that the population has been steadily aging in recent years. “We’ve been losing a lot of younger people to other areas of the country. We’re doing a lot of adult retraining,” he said. The county is home to a huge diaspora of health care workers affiliated with the Cleveland Clinic, and other players in the area’s huge medical cluster.

Stark County

2016 primary winners: Clinton, Kasich

2012: Obama 50%, Romney 49%

Population: 375,165 (2015 est.)

No party registration

A bellwether county that includes Canton, it’s part of a heavily blue collar region where Trump’s message could resonate. The county narrowly voted for Bush in 2000 but has gone Democratic in the three presidential elections since then, though never by more than 52 percent.

Pittsburgh’s Allegheny County, the second-most populous county in the state after Philadelphia, is heavily Democratic. But it’s also home to the largest concentration of registered Republicans in the state. Clinton is counting on running up big numbers in the city of Pittsburgh to offset any gains Trump may make in the suburbs, and in the more conservative surrounding southwestern Pennsylvania counties, including Westmoreland County.

One of the big four suburban collar counties ringing Philadelphia — and the fourth-most populous county in the state — Bucks is always competitive. Obama won comfortably here in 2008 but only squeaked to victory four years later. Trump appears likely to lose many moderate Republicans here but his message is also expected to appeal to white working class voters in Lower Bucks County, closer to the city.

Wilkes-Barre’s Luzerne County is traditionally Democratic territory in northeastern Pennsylvania but it’s also an aging, working-class county where Trump’s message is resonating. While Trump won every county in the state in the GOP primary, he posted his best result in Luzerne — 77 percent. While Clinton has family ties to the coal-producing region — her father was born in neighboring Scranton — mid-July polling from a GOP pollster suggests Trump is winning big here.

VIRGINIA

Henrico County

2016 primary winners: Clinton, Rubio

2012 results: Obama 55%, Romney 43%

Population: 325,155 (2015 est.)

Latest voter registration totals: 216,082
No party registration

Suburban Richmond’s Henrico County was once a reliable GOP stronghold — it went twice for George W. Bush and backed Bob McDonnell in his 2009 gubernatorial win. But its population has steadily diversified, including a surge in Asian and Latino voters, which has made it more competitive — Obama carried it twice. If Clinton can carry Henrico, it will help offset nearby Chesterfield County, the slightly larger and more Republican-oriented Richmond suburb. Her running mate Tim Kaine — the former Richmond mayor and Virginia governor — could make a difference here,

Loudoun County

2016 primary winners: Clinton, Rubio

2012: Obama 52, Romney 47

Population: 375,629 (2015 est.)

Latest voter registration totals: 230,236
No party registration

Loudoun County flipped from red to blue in 2008, when Barack Obama won it and held it in 2012. The exurban county has seen a surge in minority voters — in fact, non-Hispanic whites only constitute 62 percent of the vote, and that’s made the area more competitive. It’s also loaded with government contractors and defense companies hit hard by the sequester. But there’s a blue collar contingent too “who feel like they’re being ignored,” according to county Democratic Chairman Marty Martinez. “I would love to see Clinton or Kaine hold a truckstop rally and appeal to those guys.”

WISCONSIN

Brown County

2016 primary winners: Sanders, Cruz

2012: Romney 50%, Obama 48%

Population: 258,718 (2015 est.)

Latest voter registration totals: 146,952
No party registration

Green Bay’s Brown County voted for George W. Bush in 2000 and 2004, then Barack Obama in 2008, then flipped to Mitt Romney in 2012. It also backed Gov. Scott Walker all three times his name appeared on the ballot. The fourth-largest county in the state, it’s home to the Green Bay Packers, which are a huge economic engine. The county also features a diverse array of industries, from dairy farms and small manufacturers to major health-care companies.

Racine County
2016 primary winners: Sanders, Cruz

2012: Obama 51%, Romney 48%

Population: 195,080 (2015 est.)

Latest voter registration totals: 113,599
No party registration

Racine County, which features a significant Hispanic and African American population, is the only competitive county bordering Milwaukee — Ozaukee, Waukesha and Washington counties are reliably Republican. It voted twice for Bush and twice for Obama.
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